[linux.git] / Documentation / kbuild / llvm.rst

.. _kbuild_llvm:

==============================
Building Linux with Clang/LLVM
==============================

This document covers how to build the Linux kernel with Clang and LLVM
utilities.

About
-----

The Linux kernel has always traditionally been compiled with GNU toolchains
such as GCC and binutils. Ongoing work has allowed for `Clang
<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be
used as viable substitutes. Distributions such as `Android
<https://www.android.com/>`_, `ChromeOS
<https://www.chromium.org/chromium-os>`_, and `OpenMandriva
<https://www.openmandriva.org/>`_ use Clang built kernels.  `LLVM is a
collection of toolchain components implemented in terms of C++ objects
<https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that
supports C and the GNU C extensions required by the kernel, and is pronounced
"klang," not "see-lang."

Clang
-----

The compiler used can be swapped out via ``CC=`` command line argument to ``make``.
``CC=`` should be set when selecting a config and during a build. ::

	make CC=clang defconfig

	make CC=clang

Cross Compiling
---------------

A single Clang compiler binary will typically contain all supported backends,
which can help simplify cross compiling. ::

	ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang

``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
example: ::

	clang --target=aarch64-linux-gnu foo.c

LLVM Utilities
--------------

LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1``
to enable them. ::

	make LLVM=1

They can be enabled individually. The full list of the parameters: ::

	make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
	  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-size \
	  READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \
	  HOSTLD=ld.lld

Currently, the integrated assembler is disabled by default. You can pass
``LLVM_IAS=1`` to enable it.

Getting Help
------------

- `Website <https://clangbuiltlinux.github.io/>`_
- `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <[email protected]>
- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
- IRC: #clangbuiltlinux on chat.freenode.net
- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_

.. _getting_llvm:

Getting LLVM
-------------

- https://releases.llvm.org/download.html
- https://github.com/llvm/llvm-project
- https://llvm.org/docs/GettingStarted.html
- https://llvm.org/docs/CMake.html
- https://apt.llvm.org/
- https://www.archlinux.org/packages/extra/x86_64/llvm/
- https://github.com/ClangBuiltLinux/tc-build
- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/
Commit	Line	Data
3519c4d6 ND	1	.. _kbuild_llvm:
3519c4d6 ND	2
fcf1b6a3 ND	3	==============================
	4	Building Linux with Clang/LLVM
	5	==============================
	6
	7	This document covers how to build the Linux kernel with Clang and LLVM
	8	utilities.
	9
	10	About
	11	-----
	12
	13	The Linux kernel has always traditionally been compiled with GNU toolchains
	14	such as GCC and binutils. Ongoing work has allowed for `Clang
	15	<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be
	16	used as viable substitutes. Distributions such as `Android
	17	<https://www.android.com/>`_, `ChromeOS
	18	<https://www.chromium.org/chromium-os>`_, and `OpenMandriva
	19	<https://www.openmandriva.org/>`_ use Clang built kernels. `LLVM is a
	20	collection of toolchain components implemented in terms of C++ objects
	21	<https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that
	22	supports C and the GNU C extensions required by the kernel, and is pronounced
	23	"klang," not "see-lang."
	24
	25	Clang
	26	-----
	27
91a9d502 NC	28	The compiler used can be swapped out via ``CC=`` command line argument to ``make``.
91a9d502 NC	29	``CC=`` should be set when selecting a config and during a build. ::
fcf1b6a3 ND	30
	31	make CC=clang defconfig
	32
	33	make CC=clang
	34
	35	Cross Compiling
	36	---------------
	37
	38	A single Clang compiler binary will typically contain all supported backends,
91a9d502	39	which can help simplify cross compiling. ::
fcf1b6a3 ND	40
	41	ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
	42
91a9d502	43	``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
e30d694c	44	``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
91a9d502	45	example: ::
fcf1b6a3	46
e30d694c	47	clang --target=aarch64-linux-gnu foo.c
fcf1b6a3 ND	48
	49	LLVM Utilities
	50	--------------
	51
91a9d502 NC	52	LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1``
91a9d502 NC	53	to enable them. ::
a0d1c951 MY	54
	55	make LLVM=1
	56
91a9d502	57	They can be enabled individually. The full list of the parameters: ::
fcf1b6a3	58
91a9d502 NC	59	make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
	60	OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-size \
	61	READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \
fcf1b6a3 ND	62	HOSTLD=ld.lld
fcf1b6a3 ND	63
7e20e47c	64	Currently, the integrated assembler is disabled by default. You can pass
91a9d502	65	``LLVM_IAS=1`` to enable it.
7e20e47c	66
fcf1b6a3 ND	67	Getting Help
	68	------------
	69
	70	- `Website <https://clangbuiltlinux.github.io/>`_
	71	- `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <[email protected]>
	72	- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
	73	- IRC: #clangbuiltlinux on chat.freenode.net
	74	- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
	75	- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
	76	- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_
	77
3519c4d6 ND	78	.. _getting_llvm:
3519c4d6 ND	79
fcf1b6a3 ND	80	Getting LLVM
	81	-------------
	82
16a122c7	83	- https://releases.llvm.org/download.html
fcf1b6a3 ND	84	- https://github.com/llvm/llvm-project
	85	- https://llvm.org/docs/GettingStarted.html
	86	- https://llvm.org/docs/CMake.html
	87	- https://apt.llvm.org/
	88	- https://www.archlinux.org/packages/extra/x86_64/llvm/
	89	- https://github.com/ClangBuiltLinux/tc-build
	90	- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
	91	- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/